Ben Schleifer with the Marin Municipal Water District wades in Lagunitas Creek to do the annual coho salmon count in 2012.

Ben Schleifer with the Marin Municipal Water District wades in Lagunitas Creek to do the annual coho salmon count in 2012.

Photo: Sarah Rice, Special To The Chronicle

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Salmon migration

Salmon migration

Drought helps coho salmon set migration record

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The strange ways of Mother Nature were on display this year when a record number of Marin County's storied coho salmon migrated to the ocean, an astonishing quirk for a fishery otherwise ravaged by drought.

Almost 20,000 juvenile coho swam out of Lagunitas Creek into the ocean in the spring in the largest salmon migration that scientists have recorded since they started estimating the fish outflow in 2006.

The increased emigration of endangered fish - nearly double the record of 11,000 from two years ago - was especially baffling because the lack of rain in the winter severely reduced the number of coho that were able to lay eggs in creeks and tributaries along the Central California coast.

Eric Ettlinger, the aquatic ecologist for the Marin Municipal Water District, said the unforeseen phenomenon challenges accepted theories about the limits of coho smolt production in the sprawling watershed in western Marin.

"This was an unexpected silver lining to the dry conditions this year, and it is related to the unique habitat limitations in this watershed," Ettlinger said. "It remains to be seen how well these fish will do and whether this increase in the population can be sustained."

Drought and survival

Coho, also known as silver salmon, are born in cold freshwater rivers and streams, where they live for a year and a half before swimming to the ocean. They typically return at age 3 to where they were hatched to lay eggs and fertilize them.

The Lagunitas Creek coho swim 33 miles through the redwood- and oak-studded valley on the northwest side of Mount Tamalpais. It is the largest wild run of coho salmon and a model for fisheries restoration in the state.

Ettlinger said the outgoing fish normally gather in the late fall and winter in the lower reaches of the Lagunitas where the water is calm. The fish rest and fatten up in the remaining flood plains before going out into the ocean.

The lower creek can only support about 7,000 to 11,000 coho, he said, so the small or late-arriving fish are chased away by the resident coho. These unfortunate fish then wash out into the ocean during winter rains and die, he said.

The difference this year, he said, is that there was so little rain that thousands of young salmon were trapped in small tributaries. As a result, they were not able to make it to the lower Lagunitas, where their brethren would have driven them away.

"Because the rains came late, it prevented the fish from going down until late," Ettlinger said, adding that coho smolts were seen every day between late March and early June as they migrated past traps on Lagunitas, Olema, and San Geronimo Creeks. "It seems to have benefited the young fish."

Tough spawning run

That doesn't mean all is good for the fish. Only 206 salmon egg clusters, known as "redds," were counted in the Lagunitas watershed this year by fisheries biologists. That's well below the 20-year average of 250 redds.

"We were expecting a larger-than-average run because these were the same fish that set the previous outmigration record of 11,000 and conditions had been good in the ocean," Ettlinger said.

The spawning run was even worse in the winter south of Marin. All the creeks between the Golden Gate and Monterey Bay were blocked by sandbars until late January and February because of the lack of rain. Only a very small number of coho reached their native streams.

Biologists would be expecting a huge salmon run next year if ocean conditions were ideal for the 20,000 juvenile coho that left the Lagunitas watershed, but conditions are anything but ideal. Besides being smaller than normal, the smolts that headed out will be dealing with potentially harsh ocean conditions due to the warming pattern known as El Niño.

"It's kind of a fluke of nature that allowed those fish to survive in the first place, and now these fish are leaving during an El Niño event," said Todd Steiner, the executive director of the Salmon Protection and Watershed Network, which works with the water district on the annual salmon count and habitat protection projects. "El Niño usually means less deep-ocean upwellings and therefore less food in the ocean, so we may still end up with less fish returning."

Lagunitas history

Coho were once so numerous in Marin County that Native Americans competed with grizzly bears to catch them along the creeks and tributaries. There were so many leaping, squirming fish in the Lagunitas watershed in the past that it was said homesteaders could spear fish from docks over the water.

The spectacular coho runs began their decline after 1873, when the first of seven dams were built in the watershed, blocking 50 percent of the historical spawning habitat. Logging, development, dams and pollution also destroyed habitat all along the coast. Central California coho were listed as endangered in 2005 under the California Endangered Species Act.

The Lagunitas run is considered a bellwether of salmon health in the region because all the fish are wild, whereas a large proportion of the coho in other places are raised in hatcheries. The primary spawning grounds are also in the middle of developed communities. Some 40 percent of the coho in the watershed are hatched in tributaries surrounded by homes, golf courses, roads and horse corrals in the 9-square-mile San Geronimo Valley.

Know the coho

Range: Coho salmon are found on both sides of the North Pacific Ocean from Hokkaido, Japan, and eastern Russia, around the Bering Sea to mainland Alaska, and south all the way to Monterey Bay.

Life cycle: Anadromous, meaning adults re-enter natal freshwater streams to spawn after spending half of their three-year life cycle in the salty Pacific. Adults die within two weeks of spawning. Fry grow to 4- to 5-inches long before heading to the Pacific.

Sources: Marin Municipal Water District, National Marine Fisheries Service, ESRI, GDT and Alaska Department of Fish and Game